1. Technical Field
The present invention relates to a multimode optical fiber having a core part and a cladding part.
2. Related Art
In a multimode optical fiber for transmitting optical signals of a plurality of modes to a core part, a refractive index profile thereof is roughly classified into two kinds including a step index (SI) type and a graded index (GI) type. The optical fiber of the GI type thereof has an αth power type refractive index profile in which the central part of a core has the highest refractive index and a refractive index is gradually lowered toward the outward position in the radial direction so that a delay time difference between the modes for transmitting the optical signals is suppressed to a low level as much as possible.
The optical fiber of the GI type ordinarily includes germanium (Ge) as a refractive index adjusting additive for raising the refractive index of glass. A quantity of the germanium (Ge) is gradually increased toward the center from the outer edge of the core part.
Further, it has been considered to use a multimode optical fiber of the GI type including fluorine (F) as a refractive index adjusting additive for lowering the refractive index of glass. A quantity of the fluorine is gradually increased toward the outward position in the radial direction from a center (see non-Patent Document 1).
Further, it has been known to use a single mode optical fiber in which a core is composed of pure silica glass and fluorine is added to a cladding to lower a refractive index (for instance, see non-Patent Document 2). This optical fiber is called a pure silica core fiber (Z fiber) and has hydrogen resistant characteristics, radiation resistant characteristics and low attenuation characteristics.
[Non-Patent Document 1] International Wire & Cable Symposium Proceedings 1984 (244 to 250 pages)
[Non-Patent Document 2] Lightwave Technology, Vol LT-4, No. 8
When submarine resources are searched, a temperature is measured by a Fiber-Optic Distributed Temperature Sensing System (DTS) using the multimode optical fiber of the GI type. In this measuring method, lights to be measured are allowed to be incident on the installed optical fiber to detect the intensity ratio for Stokes' lights and anti-Stokes' lights of Raman scattered lights returning to an incident end, so that the temperature at a measuring position is measured based on a measuring time.
In such a measurement of temperature, when the optical fiber including germanium in a core to adjust a refractive index is used, since attenuation is increased by hydrogen in the submarine resources, a highly accurate measurement cannot be carried out. For instance, when the attenuation is increased by the hydrogen in a wavelength band of around 1400 nm, the influence of the increase of the attenuation is also given to the anti-Strokes' lights (1455 nm) and thereby generates a difference in increase of the attenuation from that of the Stokes' lights (1633 nm) apart from the 1400 nm band. The difference in increase of the attenuation causes the temperature detecting accuracy of the DTS to be deteriorated.
It has been known to use the optical fiber in which the surface of the multimode optical fiber of the GI type with germanium added to the core is coated with carbon to prevent the entry of hydrogen. However, in this optical fiber, under an environment of high temperature (for instance, 120° C. or higher), the entry of hydrogen cannot be prevented.
As compared therewith, in the optical fiber which is disclosed in the non-Patent Document 1 and has fluorine added to the core and the cladding, the increase of the attenuation by hydrogen does not arise, so that a highly accurate measurement can be achieved. However, it has been extremely difficult to continuously change the quantity of the fluorine in the radial direction to form an αth power type refractive index profile with good accuracy.
In the single mode fiber (pure silica core fiber) disclosed in the non-Patent Document 2, since the diameter of the core is small, a light having a large power cannot be inputted. Further, a numerical aperture NA is small, the probability of capture of the Raman scattered lights is reduced to lower an S/N and deteriorate the temperature detecting accuracy of the DTS. If the incident power is increased, an inductive Raman scattering is caused to deteriorate the S/N and the lights cannot be transmitted thereby.